An image forming apparatus using electrophotography may include a copier, printer, and facsimile, for example.
Such an image forming apparatus may have a charging unit, by which a photoconductive member may be charged with a substantially uniform voltage (or potential). Then, a light beam corresponding to a document image may be irradiated on the charged surface of the photoconductive member to form an electrostatic latent image, corresponding to the document image.
Such an electrostatic latent image may be developed as a visible image (e.g., a toner image) with an image developer having carrier particles and toner particles.
For example, in a case of a developing process using a magnetic brush, an image developer having magnetic carrier particles and toner particles (e.g., color resin) may be used to develop an electrostatic latent image formed on a photoconductive member.
Such a developed toner image on a photoconductive member may be transferred to a transfer sheet, and then fixed on the transfer sheet by a fixing unit, which may apply heat to the transfer sheet.
In such an image forming apparatus using electrophotography, toner particles in the image developer may be consumed gradually during a process of developing latent images with toner particles, by which a ratio of carrier and toner in image developer may change over time.
If a toner ratio in the image developer is reduced, a developed image concentration may also be unfavorably reduced. Accordingly, toner particles may need to be refilled at a given timing.
However, if a refilling amount of toner particles is too great, an image quality on a transfer sheet may be degraded, which may be observed as an increased image concentration, or an unintended image on a transfer sheet.
Accordingly, a toner ratio in the image developer may need to be maintained at a given preferable level to continuously obtain a higher quality image having a preferable concentration level.
In view of such background, methods of automatically controlling a toner ratio in an image developer have been devised.
For example, one method is to determine a toner ratio in an image developer by detecting an image concentration of a test pattern formed on a photoconductive member with an optical detector. Another method is to determine a toner ratio in an image developer by measuring a magnetic permeability of the image developer.
Based on a detection signal obtained by such methods, a controller may instruct a toner supply mechanism, provided to a developing unit, to supply toner particles into a developing unit to maintain a toner ratio in the image developer at a given preferable level.
Although such methods may be employed for an image forming apparatus (e.g., copier, printer, facsimile) using electrophotography to maintain a toner ratio in image developer at a given preferable level, an image quality formed on a transfer sheet may be degraded when an image forming apparatus conducts image forming operations (e.g., copying, printing) for a relatively greater number of times.
Such image quality degradation may be caused by a lifetime of the image developer, for example. As mentioned above, the image developer in a developing unit may have carrier particles and toner particles, wherein a ratio of toner in the image developer may be several percent. Accordingly, the image developer may consist mostly of carrier particles while including a small percentage of toner particles.
As mentioned above, toner particles may be gradually consumed during a process of developing latent images with toner particles while carrier particles may not be consumed in such a developing process. The carrier particles may be re-circulated and reused in a developing unit. With such repeated use of carrier particles, carrier particles may be aged and degraded.
Such an image developer may be aged and degraded as described below. For example, a surface of carrier particles may be covered with toner particles by conducting a developing process for a greater number of times, or a surface of carrier particles may be damaged by conducting a developing process for a greater number of times.
As for such an image developer having a given lifetime, a replacement of image developer may be conducted when a service person conducts a maintenance work for an image forming apparatus, for example.
However, such replacement work may take some time, which may not a favorable aspect for a user of image forming apparatus.
In view of such background, a recent market demand may include a reduction of down time of an image forming apparatus caused by maintenance work such as replacement of image developer. Furthermore, some users may be demanding a substantial elimination of replacement work of image developer.
Furthermore, carrier particles may be agitated in a developing unit for transporting carrier particles in the developing unit. Accordingly, a surface of the carrier particles may be damaged by physical stress, by which charge-ability or electric resistance of the carrier particles may be degraded. Furthermore, toner particles or additives may adhere on the surface of the carrier particles and may form a film on the carrier particles.
With such degradation, carrier particles and toner particles may not be charged at a normal level, by which an unfavorable phenomenon may occur. For example, toner sputtering, unintended image formation, and/or carrier particle adhesion may occur.
As for a reduction of down time of an image forming apparatus, caused by replacement work of image developer, the following related art has been devised.
One related art apparatus using electrophotography has a developing unit, and a supply unit. Such a supply unit may supply a given amount of carrier particles to the developing unit when a given developing time has passed or when a given amount of copying operations has been conducted.
In such an apparatus, a condition of the image developer in the developing unit may be maintained at a given level by a given process such as “refilling fresh carrier particles into the developing unit in addition to refill toner particles, consumed by image forming operation,” “ejecting excessive image developer from a developing unit,” and “replacing degraded image developer from a developing unit,” for example.
Such a method may be termed a “trickle developing system,” which may be used in a developing unit for an image forming apparatus such as a copier using electrophotography.
In such a trickle developing system, fresh carrier particles may be refilled into a developing unit while separately refilling toner particles consumed by image forming operata ions.
In such trickle developing system, an excessive amount of image developer in the developing unit may be overflowingly ejected from an ejecting port, provided in a wall face of the developing unit, and such overflowed image developer may be recovered by a recovery unit.
Such refilling of carrier particles and ejection of degraded image developer may be repeated in the developing unit. With such a refilling and ejection process, degraded image developer may be replaced by fresh toner particles and carrier particles supplied to the developing unit.
With such a process, a charging ability of the image developer may be maintained at a given level, and thereby a degradation of image quality may be suppressed or reduced.
Furthermore, in a developing unit of another related art apparatus, a refilling amount of toner and an ejection amount of image developer may be controlled by detecting an image developer volume in an image developer container.
Furthermore, in a developing unit of another related art apparatus, a relationship between an aging speed of carrier particles and a charge-ability of toner particles in a housing may be set as a mathematical function. Carrier particles may be added into the housing at a given timing based on referring to the mathematical function. With such a mathematical function setting, a lifetime of the image developer and a lifetime of the image forming apparatus (e.g., printer) may be set to a substantially equal time.
Furthermore, in another related art apparatus, a refilling amount of carrier particles may be changed (or adjusted) based on a toner consumption amount. For example, if a toner consumption amount becomes greater, the refilling amount of carrier particles may be increased. Accordingly, carrier particles may be refilled by checking a degradation level of the carrier particles, wherein such a degradation level may become different depending on the toner consumption amount.
However, a degradation level of the carrier particles may not be determined only by a toner consumption amount, and a toner ratio in the image developer may not be a stable level when refilling the carrier particles. Therefore, an unfavorable change may occur to a toner and carrier ratio in a developing unit if a toner refilling amount and a carrier refilling amount may be determined only by the toner consumption amount.
Furthermore, in another related art apparatus, a degradation level of an image developer may be detected and then image developer may be replaced, in which a total amount of image developer in a developing unit may be replaced with fresh image developer.
Accordingly, such total replacement of the image developer may be different from a trickle developing system, and in such a total replacement method, a down time caused by replacement work of image developer may become relatively longer, which may not be preferable.
In background art apparatuses, a given amount of image developer may be refilled based on a number of printed sheets, or an image developer may be refilled by mixing carrier particles with refilling toner particles.
Such methods may be set based upon an assumption that carrier particles may degrade at a given timing, which may be set in advance, and may refill fresh image developer or carrier particles when such a given timing has elapsed.
Accordingly, if an actual degradation timing of image developer is later than an assumed degradation timing, an image developer that is still usable for image forming may be replaced from a developing unit with fresh image developer and fresh carrier particles, which may not be preferable from a viewpoint of saving material.
Furthermore, if an actual degradation timing of image developer is earlier than an assumed degradation timing, fresh image developer and carrier particles may not be refilled at a correct timing, by which image quality may degrade.
Accordingly, in some cases, a refilling amount or replacement amount of image developer and carrier particles may not match a degradation level of the image developer and carrier particles, by which a degradation of image quality may not be effectively suppressed or reduced, and a lifetime of image developer may not be effectively extended.
For example, a system, which may refill carrier particles by mixing carrier particles to refilling toner particles, may have a drawback when images having a lower image area ratio are printed for a greater number of times. In such an image forming process, toner particles may not be refilled for a longer period of time, and thereby carrier particles may be agitated in a developing unit without refilling the developing unit with fresh carrier particles for a longer period of time, by which carrier particles in the developing unit may degrade significantly.
Such a system, in which carrier particles may be refilled by mixing carrier particles with refilling toner particles when refilling toner particles, may have another drawback when images having a higher image area ratio are printed for a greater number of times. In such an image forming process, a greater amount of toner particles may be refilled due to a consumption of a greater amount of toner particles, and also a greater amount of carrier particles may be refilled at the same time, by which the amount of refilling carrier particles in the developing unit may exceed a required refilling amount of carrier particles, which may not be preferable from a viewpoint of saving carrier particles.